This invention relates generally to radio frequency receivers and more methods for reducing DC offsets in such receivers.
As is known in the art, received radio frequency signals are converted to baseband using various receivers. With a homodyne receiver, the received radio frequency signal is mixed with the local oscillator whose frequency is equal to carrier frequency of the received radio frequency signal to translate the carrier frequency to DC and thereby provide xe2x80x9cdirect conversionxe2x80x9d of the modulation on the received radio frequency signal to a modulation at DC. Hence, a homodyne receiver is sometimes referred to as a direct conversion receiver.
While such direct conversion receivers offer the advantage of reduced cost, DC offset associated with such receivers presents a major problem for receiver performance. More particularly, DC offset is produced from the homodyning. The level of the DC offset may be significantly larger than the desired signal, i.e., modulation, to be demodulated. Thus, DC offset compensation techniques are typically required. To insure-flexibility for different operating conditions, DC offset compensation can be part of the digital baseband portion of the receiver, preferably a part of the digital signal processing (DSP) firmware. One application of direct conversion receivers is in mobile or cellular communication systems. In such systems, the radio channels received signals also suffer from intersymbol interference (ISI) caused by multipath radio propagation and transmitter and/or receiver filtering.
More particularly, various DC offset compensation techniques, for both analog and digital domain, have been suggested. With regard to the Global System for Mobile Communications (GSM) environment, these suggested techniques are largely dependent on the sources of DC offset since they result in different levels of DC offset compared to the desired signal level. As discussed in an article entitled xe2x80x9cDesign considerations for direct-conversion receivers,xe2x80x9d by B. Rezavi, published in IEEE Transactions on circuits and systemsxe2x80x94II: analog and digital signal processing, June 97, pp 428-435, two major mechanisms causing DC offset in direct conversion receivers are: Local Oscillator (LO) isolation to the receiver""s Low Noise Amplifier (LNA) and mixer inputs; and interference leakage to the LO (i.e., self mixing). The level of DC offset in this case is much larger than the level of the desired signal.
Various techniques have been suggested to remove DC offset generated by these two major mechanism. Included in these techniques are: AC coupling; Offset cancellation using capacitor; Sample mean (average) estimation; Adaptive DC offset compensation for burst mode operation; and Re-integration.
AC coupling, requires high-pass filter with corner frequency 0.1% of the data rate, which is less that 270 Hz for GSM. Problems associated with this approach are elimination of signal content around DC, group delay characteristic of the filter and settling time in Time Division Multiple Access (TDMA) environment.
With offset cancellation using a capacitor for TDMA systems, the offset in the receive path can be stored on a capacitor during the idle mode and subtracted from the signal during actual reception in the burst. The major issues with this technique are kT/C noise and problems when interferer is stored along with other offsets.
With the sample mean (average) estimation based technique, such technique includes averaging over sufficiently long period and subtraction, as described by A. Bateman and D. Haines, in xe2x80x9cDirect conversion transceiver design for compact low-cost portable mobile radio terminals,xe2x80x9d in Proc. VTC""89, pp 57-62. In TDMA systems, averaging is usually performed over the burst duration. The issues associated with this approach are: it does not address DC offset changes within burst; and it may introduce some bias since burst does not have zero DC component due to different number of zeros and ones in the data stream. Besides its simplicity this method has some desirable statistical properties. Sample mean estimate is optimal DC offset estimate in zero-mean noise with Gaussian probability density function in minimum mean-squared error sense (also minimum variance unbiased estimate) and maximum likelihood sense. Even when the probability density function (pdf) of the noise is not known, the signal average is the best linear unbiased estimate, see Fundamentals of statistical signal processing: estimation theory, Prentice Hall, 1993 by S. Kay.
Adaptive DC offset compensation for burst mode operation is presented in a paper entitled xe2x80x9cAdaptive DC offset compensation algorithm for burst mode operated direct conversion receivers,xe2x80x9d by S. Sampei and K. Feher, Proc. VTC""92, pp 93-96. This approach utilizes the known bits from the preamble to acquire DC offset, typically 3-5 bits. Simulations have shown that the technique is efficient in the cases where the DC offset ratio (amplitude ratio of DC offset to the maximum amplitude of the transmitted symbol) is less than 40%. Degradation of the performance is up to 1.5 dB. A similar approach has been presented by J. Bergmans, Digital baseband transmission and recording, Kluwer Academic Publishers, Section 8.8.2. in a more general communication scenario of continuous reception. In essence it is a form of dynamic loop tracking DC offset.
Digital compensation based on Least Mean Square (LMS) adaptive algorithm has been presented in by J. Cavers and M. Liao, in xe2x80x9cAdaptive compensation for imbalance and offset losses in direct conversion receivers,xe2x80x9d IEEE Transactions on Vehicular Technology, November 93, pp 581-588. Such paper describes models and theoretical development of receiver and transmitter compensation (modulator and demodulator). The least mean square (LMS) algorithm has been applied to set the parameters of the compensating circuit (compensates for In-Phase (I) and Quadrature (Q) gain and phase imbalance and DC offset). Effectively the system has three adaptive coefficients. A drawback of the algorithm is the long convergence time and sensitivity to the selection of LMS step-size parameters.
The re-integration approach is presented in a paper by B. Lindquist, M. Isberg and P.Dent, entitled xe2x80x9cA new approach to eliminate the DC offset in a TDMA direct conversion receiver,xe2x80x9d In Proc. VTC""93, pp 754-757 [7]. The idea is to differentiate signal, digitize it and then re-integrate, thus eliminating DC component. It is based on adaptive delta modulation and since there is no time constant it targets TDMA direct conversion receivers. Simulation results presented in the paper indicate signal to noise ratio (SNR) degradation of 1 dB in static channel for the Bit Error Rate (BER) range 1% to 0.1%.
In a GSM system, the whole burst is stored and the all-digital techniques described above may be adapted to extract DC offset. Thus, referring to FIG. 1, the data receiver stores the burst of data, r(k), where k=1 . . . N and N is the number of samples in the burst. Each burst includes a mid-amble having a known sequence of bits disposed between data, (i.e., information bits) as shown. Such known sequence of bits is used to aid in equalization and more particularly for enabling computation of the channel impulse response (CIR). As shown in FIG. 1, an estimate of the DC offset, Â, is calculated. The estimated DC offset, Â, where k=1 . . . N, is subtracted from the received burst. The result, r(k)xe2x88x92Â, where k=1 . . . N, is processed to find an estimate of the channel impulse response (CIR), ĥ. The estimate of the channel impulse response (CIR), ĥ, can be obtained by cross-correlating [r(k)xe2x88x92{right arrow over (A)}] with the known mid-amble bit sequence.
The complexity of the DC offset cancellation in GSM system is related to other signal processing functions performed in baseband (synchronization, equalization). Residual DC offset may affect the performance of data receiver.
In accordance with the present invention, a method is provided for reducing DC offset from a received signal. The method includes jointly estimating such DC offset and a channel impulse response, ĥ, and reducing the DC offset in accordance with the estimated DC offset and the estimated channel impulse response, ĥ.
In accordance with another feature of the invention, a communication system is provided wherein information is transmitted through a channel having a discrete channel impulse response h(k), where k is a time index, to produce at an output of the channel, a signal, r(k), where:       r    ⁢          (      k      )        =      A    +                  ∑        n            ⁢                        b          ⁢                      (            n            )                          ⁢                  h          ⁢                      (                          k              -              n                        )                                +          N      ⁢              (        k        )            
where:
A is DC offset;       ∑    n    ⁢            b      ⁢              (        n        )              ⁢          h      ⁢              (                  k          -          n                )            
is a modulated signal transmitted over the channel 42;
b(n) are transmitted data symbols; and
N(k) is additive noise.
The system includes a receiver for receiving the transmitted information. The receiver has a processor programmed to solve the following equations simultaneously:                     ∂                  f          ⁡                      (                          e              ⁡                              (                k                )                                      )                                      ∂                  A          ^                      =    0                      ∂                  f          ⁡                      (                          e              ⁡                              (                k                )                                      )                                      ∂                  h          ^                      =    0  
where:
f(e(k)) is a function, usually quadratic, of the estimation error, e(k), where
e(k) is the difference between the received signal and an estimate of the received signal; i.e.,       e    ⁢          (      k      )        =            r      ⁢              (        k        )              -          A      ^        -                  ∑        n            ⁢              [                              b            ⁢                          (              n              )                                ⁢                      h            ^                    ⁢                      (                          k              -              n                        )                          ]            
In accordance with another feature of the invention, a communication system is provided wherein information is transmitted through a channel as a series of bursts, each burst having a predetermined series of bits and a series of information bits. The system includes a receiver for receiving the transmitted information. The receiver has a processor programmed to simultaneously solve the following equations from: (a) the predetermined series of bits; (b) a tentative decision of the information bits; or, a combination of the predetermined series of bits and the tentative decision of the information bits:                     ∂                  f          ⁡                      (                          e              ⁡                              (                k                )                                      )                                      ∂                  A          ^                      =    0                      ∂                  f          ⁡                      (                          e              ⁡                              (                k                )                                      )                                      ∂                  h          ^                      =    0  
where:
f(e(k)) is a function, usually quadratic, of the estimation error, e(k), where
e(k) is the difference between the received signal and an estimate of the received signal; i.e.,             e      ⁡              (        k        )              =                  r        ⁡                  (          k          )                    -              A        ^            -                                    ∑            [                    n                ⁢                  b          ⁡                      (            n            )                          ⁢                              h            ^                    ⁡                      (                          k              -              n                        )                                ]